Hi.
I am wondering about something. Sometimes when the battery is connected to the circuit, a small (sometimes not so small) spark can be seen. Is that a problem or not? I have not taken any notice of it so far, thinking that it also does it when i plug in my computer to the wall so i guess it is okay...... or is it?
Cheers
Sounds fairly normal to me, it indicates that your circuit is drawing current. If the sparks get too big for comfort though, I would use a multimeter to check how much current is being drawn, and reduce it somehow (or get a more capable power source). Just out of curiosity, do you have any inductive loads in your circuit, like motors or solenoids? Those draw lots of current when they're first turned on.
Yes i do, but they are of cause connected to a motor controller and are not "active" until they are supplied with a PWM from the Arduino.
It could also be an empty capacitor in the power supply filling up and drawing a momentary surge.
The sparks can cause a problem after many sparks. Melting and vaporization of the plug will get worse as the weeks go by and hundreds of sparks occur. Use a switch so the plug is not eroded.
It is capacitors charging. You don't say what you are connecting to the battery.
Thank you all for the reply.
I am connecting a OSMC motor board which of cause is connected to a brushed dc motor.
I will replace my dodgy wire-setup with a switch 
Thanks a bunch
The problem with such sparks is that create large noise spikes which can be destructive
to sensitive circuitry. Proper electrical switch gear is designed to bring the metal contacts
together quickly and thus limit the duration of a contact spark.
This is why you should switch off the switch, plug in the circuit, then switch on the switch
(if at all possible).
And the switch needs to be big enough to handle the current inrush, which as has been
mentioned depends a lot on the capacitance being charged. For big capacitors it can
be a good idea to have a two-phase switch-on, a first switch with a series power resistor
that limits the inrush current but allows the capacitor to charge most of the way, then
a main switch to provide a proper connection. Much less stress on everything!
That sounds responsible with the power resistor.
When i turn the power on (connect the battery) with the motor at standstill, i am using 0.3A @ 24V. So what guidelines would i use to choose a power resistor?
Cheers
The precharge resistor can be a 10 watt power resistor from Radio Shack with 8 ohms or a light bulb. The .3 amps at 24 volts are multiplied to get the power
.3 x 24 = 7.2 watts.
The light bulb can be good to see the precharge light up the bulb and then the light bulb goes dark when precharging is done.
It would be helpful to know the capacitance of your load. Here is an example :
220uF x 8 ohms = 1.7 milliseconds RC time constant to charge to 63% of the full voltage.
220uF x 8000 ohms = 1.7 seconds to precharge, then throw the main power switch to bypass the resistor.
Having a 2 stage turn-on for a 220uF capacitor seems like overkill.
AmbiLobe, how do you explain the millions upon millions of power supplies that don't use such a two stage turn-on process?